Kerogen

Kerogen is solid, insoluble organic matter in sedimentary rocks. Consisting of an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter by 10,000 fold. It is insoluble in normal organic solvents and it does not have a specific chemical formula. Upon heating, kerogen converts in part to liquid and gaseous hydrocarbons. Petroleum and natural gas form from kerogen. Kerogen may be classified by its origin: lacustrine (e.g., algal), marine (e.g., planktonic), and terrestrial (e.g., pollen and spores). The name 'kerogen' was introduced by the Scottish organic chemist Alexander Crum Brown in 1906, derived from the Greek for 'wax birth' (Greek: κηρός 'wax' and -gen, γένεση 'birth'). Kerogen is solid, insoluble organic matter in sedimentary rocks. Consisting of an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter by 10,000 fold. It is insoluble in normal organic solvents and it does not have a specific chemical formula. Upon heating, kerogen converts in part to liquid and gaseous hydrocarbons. Petroleum and natural gas form from kerogen. Kerogen may be classified by its origin: lacustrine (e.g., algal), marine (e.g., planktonic), and terrestrial (e.g., pollen and spores). The name 'kerogen' was introduced by the Scottish organic chemist Alexander Crum Brown in 1906, derived from the Greek for 'wax birth' (Greek: κηρός 'wax' and -gen, γένεση 'birth'). Increasing production of hydrocarbons from shale has motivated a revival of research into the composition, structure, and properties of kerogen. Many studies have documented dramatic and systematic changes in kerogen composition across the range of thermal maturity relevant to the oil and gas industry. Analyses of kerogen are generally performed on samples prepared by acid demineralization with critical point drying, which isolates kerogen from the rock matrix without altering its chemical composition or microstructure. Kerogen is formed during sedimentary diagenesis from the degradation of living matter. The original organic matter can comprise lacustrine and marine algae and plankton and terrestrial higher-order plants. During diagenesis, large biopolymers from, e.g., proteins and carbohydrates in the original organic matter decompose partially or completely. (This breakdown process can be viewed as the reverse of photosynthesis). These resulting units can then polycondense to form geopolymers. The formation of geopolymers in this way accounts for the large molecular weights and diverse chemical compositions associated with kerogen. The smallest units are the fulvic acids, the medium units are the humic, and the largest units are the humins. This polymerization usually happens alongside the formation and/or sedimentation of one or more mineral components resulting in a sedimentary rock like oil shale. When kerogen is contemporaneously deposited with geologic material, subsequent sedimentation and progressive burial or overburden provide elevated pressure and temperature owing to lithostatic and geothermal gradients in Earth's crust. Resulting changes in the burial temperatures and pressures lead to further changes in kerogen composition including loss of hydrogen, oxygen, nitrogen, sulfur, and their associated functional groups, and subsequent isomerization and aromatization Such changes are indicative of the thermal maturity state of kerogen. Aromatization allows for molecular stacking in sheets, which in turn drives changes in physical characteristics of kerogen, such as increasing molecular density, vitrinite reflectance, and spore coloration (yellow to orange to brown to black with increasing depth/thermal maturity). During the process of thermal maturation, kerogen breaks down in high-temperature pyrolysis reactions to form lower-molecular-weight products including bitumen, oil, and gas. The extent of thermal maturation controls the nature of the product, with lower thermal maturities yielding mainly bitumen/oil and higher thermal maturities yielding gas. These generated species are partially expelled from the kerogen-rich source rock and in some cases can charge into a reservoir rock. Kerogen takes on additional importance in unconventional resources, particularly shale. In these formations, oil and gas are produced directly from the kerogen-rich source rock (i.e. the source rock is also the reservoir rock). Much of the porosity in these shales is found to be hosted within the kerogen, rather than between mineral grains as occurs in conventional reservoir rocks. Thus, kerogen controls much of the storage and transport of oil and gas in shale. Kerogen is a complex mixture of organic chemical compounds that make up the most abundant fraction of organic matter in sedimentary rocks. As kerogen is a mixture of organic materials, it is not defined by a single chemical formula. Its chemical composition varies substantially between and even within sedimentary formations. For example, kerogen from the Green River Formation oil shale deposit of western North America contains elements in the proportions carbon 215 : hydrogen 330 : oxygen 12 : nitrogen 5 : sulfur 1. Kerogen is insoluble in normal organic solvents in part because of its high molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right temperatures in the earth's crust, (oil window c. 50–150 °C, gas window c. 150–200 °C, both depending on how quickly the source rock is heated) some types of kerogen release crude oil or natural gas, collectively known as hydrocarbons (fossil fuels). When such kerogens are present in high concentration in rocks such as organic-rich mudrocks shale, they form possible source rocks. Shales that are rich in kerogen but have not been heated to required temperature to generate hydrocarbons instead may form oil shale deposits. The chemical composition of kerogen has been analyzed by several forms of solid state spectroscopy. These experiments typically measure the speciations (bonding environments) of different types of atoms in kerogen. One technique is 13C NMR spectroscopy, which measures carbon speciation. NMR experiments have found that carbon in kerogen can range from almost entirely aliphatic (sp3 hybridized) to almost entirely aromatic (sp2 hybridized), with kerogens of higher thermal maturity typically having higher abundance of aromatic carbon. Another technique is Raman spectroscopy. Raman scattering is characteristic of, and can be used to identify, specific vibrational modes and symmetries of molecular bonds. The first-order Raman spectra of kerogen comprises two principal peaks; a so-called G band (“graphitic”) attributed to in-plane vibrational modes of well-ordered sp2 carbon and a so-called D band (“disordered”) from symmetric vibrational modes of sp2 carbon associated with lattice defects and discontinuities. The relative spectral position (Raman shift) and intensity of these carbon species is shown to correlate to thermal maturity, with kerogens of higher thermal maturity having higher abundance of graphitic/ordered aromatic carbons. Complementary and consistent results have been obtained with infrared (IR) spectroscopy, which show that kerogen has higher fraction of aromatic carbon and shorter lengths of aliphatic chains at higher thermal maturities. These results can be explained by the preferential removal of aliphatic carbons by cracking reactions during pyrolysis, where the cracking typically occurs at weak C-C bonds beta to aromatic rings and results in the replacement of a long aliphatic chain with a methyl group. At higher maturities, when all labile aliphatic carbons have already been removed—in other words, when the kerogen has no remaining oil-generation potential—further increase in aromaticity can occur from the conversion of aliphatic bonds (such as alicyclic rings) to aromatic bonds. IR spectroscopy is sensitive to carbon-oxygen bonds such as quinones, ketones, and esters, so the technique can also be used to investigate oxygen speciation. It is found that the oxygen content of kerogen decreases during thermal maturation (as has also been observed by elemental analysis), with relatively little observable change in oxygen speciation. Similarly, sulfur speciation can be investigated with X-ray absorption near edge structure (XANES) spectroscopy, which is sensitive to sulfur-containing functional groups such as sulfides, thiophenes, and sulfoxides. Sulfur content in kerogen generally decreases with thermal maturity, and sulfur speciation includes a mix of sulfides and thiophenes at low thermal maturities and is further enriched in thiophenes at high maturities.